Embodiments according to the invention relate to heterostructure semiconductor components and in particular to a semiconductor structure, a transistor and a method for manufacturing a semiconductor structure.
With the heterostructure field effect transistor, the current flowing through the component between the two ohmic contacts, the source and drain, is ideally controlled in a powerless method by means of a voltage applied to a non-conducting metal semiconductor contact (gate). In contrast with homogeneous field effect transistors, an HFET has a heterogeneous material system. This utilizes the fact that a potential pot having discrete energy levels and representing the channel is formed at the interface when combining two suitable semiconductor materials having different band gap energies. The advantage lies in the separation of the charge carriers and the doped region. Due to the fact that the current transport takes place in the undoped material, high mobilities and thus good high-frequency properties can be achieved. The charge carrier concentration in the channel is modulated by the applied gate voltage.
Heterostructure field effect transistors (HFETs), also known as high electron mobility transistors (HEMTs) based on nitrides of group III, are capable of handling higher powers than comparable devices based on other semiconductor material systems at high frequencies, usually 1-100 GHz. These abilities are a result of the unique material properties of group III nitrides, which have a broad band gap, a large breakdown field and a high charge carrier saturation velocity. By using the binary compounds aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN) and their alloys (ternary AlGaN, GaInN, AlInN as well as quaternary AlGaInN), a large bandwidth of desired material properties can be covered. Established applications cover mainly the range between 2 and 12 GHz. At higher frequencies, for example, around 35 GHz and 94 GHz for radar with a high spatial resolution, nitride HFETs can offer power levels not accessible for solid-state systems in the past. These applications necessitate a small gate length and thus thin barrier layers in combination with high surface charge carrier densities and high electron mobilities.
The best researched heterostructure for use with HFETs is the AlGaN/GaN system. Although the material growth and the component technology for AlGaN/GaN have reached a rather high level of maturity, the stress in the AlGaN layer is an inherent disadvantage, which results in physical limits for component design. This problem is especially serious in implementation of heterostructures with an ace electron density of more than 1.5×1013 cm−2, which is almost impossible due to stress relaxation and the limited lifetime of AlGaN/GaN. Another disadvantage of AlGaN/GaN associated with this is the difficulty of producing heterostructures, which have a high face electron density at the same time and a thin barrier, such as that necessitated for effective scaling with the gate length for fast transistors.
In recent reports, the AlGaN barrier has been replaced by the compound AlInN with an indium concentration of approx. 18% (see, for example, “Gonschorek et al., Appl. Phys. Lett. 89, 062106, (2006)”) and “Xie et al., Appl. Phys. Lett. 91, 132116, (2007)”). With this composition, the in-plane lattice constant of AlInN is the same as that of GaN, so that an AlInN/GaN heterostructure is lattice-matched and stress-free. Because of the high aluminum content and consequently the high spontaneous polarization of lattice-matched AlInN, a barrier layer thickness of less than 10 nm is sufficient to obtain a high face electron density. However, AlInN has alloy non-miscibility and consequently has a poor quality as a material. Simple AlInN/GaN heterostructures have only a fraction of the electron mobility of typical AlGaN/GaN wafers. Furthermore, ohmic contacts with AlInN-based structures, which are essential for HFET operation, have terminal resistances that are higher by several factors than those with AlGaN-based references.
The electron mobility with AlInN-based heterostructures can be greatly improved by inserting an AlN intermediate layer between the GaN channel and the lattice-adapted barrier layer (“Gonschorek et al., Appl. Phys. Lett. 89, 062106, (2006)”). The AlN serves as a spacer separating the two-dimensional electron gas (2DEG) in the channel from the AlInN barrier layer and permitting an improved interfacial roughness in comparison with a direct AlInN/GaN sequence. Nevertheless, even optimized AlInN/AlN/GaN structures only have mobility values substantially lower than those of AlGaN/GaN or AlGaN/AlN/GaN structures.
Additional examples of heterostructures and their applications are presented below. For example, US Patent 2002/0058349 A1 describes a method for producing nitride-based heterostructure components by using quaternary layers containing AlInNGaN.
In addition, US Patent 2008/0054303 A1 describes a group III nitride-based field effect transistor, which has an improved power characteristic due to manipulation of the relationship between the in-plane lattice constants of the interface of material layers.
In addition, US 2006/0197109 A1 describes a transistor having a high electron mobility with a GaN channel structure having a very thin (Al, In, Ga)N subchannel layer arranged between a first GaN channel layer and a second GaN channel layer to bring about a band bending induced by the piezoelectric and spontaneous charge carriers in association with the (Al, In, Ga)N subchannel layer. Two or more channels are therefore formed to improve the linearity of the transistor.
The publication “Liu et al., Appl. Phy. Lett. 86, 223510 (2005) Demonstration of undoped quaternary AlInGaN/GaN heterostructure field effect transistor on sapphire substrate” demonstrates an undoped AlInGaN heterostructure field effect transistor on a sapphire substrate.
In addition, the publication “Liu et al., Jpn. J. Appl. Phys. 45, 5728-5731 (2006) Novel quaternary AlInGaN/GaN heterostructure field effect transistors on sapphire substrate” discloses an undoped quaternary AlInGaN/GaN heterostructure field effect transistor with different molar amounts of aluminum produced on a sapphire substrate.
In addition, “Adivarahan et al., IEEE Trans. Electron Devices 55, 495-499 (2008) Double-recessed high-frequency AlInGaN/GaN metal oxide double heterostructure field effect transistors” discloses a low-threshold AlInGaN/InGaN/GaN metal oxide semiconductor double heterostructure field effect transistor for high-frequency applications.